Embodiments of the present invention relate to water-entrained-polyimide chemical compositions comprising stable aqueous-based colloidal solutions for use in high-performance composite fabrication and methods of making the same.
It is well known in the art that polyimides exhibit properties desirable for high-temperature applications such as those demanded in the aerospace industry. Existing materials are primarily based on Polymerization of Monomeric Reactants (PMR) chemistry and are with a few exceptions, only suitable for prepreg. Additionally, these materials are plagued with various deficiencies. For example, PMR-15, which is described in U.S. Pat. No. 3,745,149, contains a known carcinogen 4,4-methylene dianiline (MDA), is prone to microcracking, difficult to handle during processing and has a short shelf life. Many PMR materials cannot be used for thick composites due to unwanted reaction byproducts.
Past methods, based primarily on PMR chemistry, of synthesizing polyimide oligomers require the use of either dimethylformamide (DMF) or an alcohol-based solvent. The use of DMF leads to processing difficulties due to the need for removing the DMF prior to the final cure. The difficulty in removing DMF prompted the utilization of alcohol-based systems for synthesizing polyimide oligomers (e.g. RP-46, PMR-II-50, PETI-330 and AFR-PE-4). However, the use of alcohol-based solvents in the synthesis of polyimides also has shortcomings. In particular, dissolution of the reactants in alcohol-based systems often leads to unwanted side reactions between the alcohol and the monomers and/or oligomers used in the reaction. Specifically, dissolution in alcohol, according to PMR chemistry, forms half esters that interfere with the formation of polyimides. These unwanted byproducts have an adverse impact on the properties of the finished products.
Accordingly, there continues to be a need for an aqueous-based system for polyimide oligomers suitable for high-performance composites that mitigate health and toxicity problems, are easily processed and exhibit improved thermo-oxidative stability. In particular, the aerospace industry presently has a need for composites and laminates that can be safely produced at a low cost and provide long-term thermal protection for temperatures up to 700° F.